If you were in North America (with raccoons, possums, weasels etc) I would say you are probably dreaming about your run being totally predatorproof; but I am guessing you're maybe in the UK? and if so, if dogs and foxes TRULY TRULY TRULY cannot possibly get into your yard, then maybe it really *is* proof against everything but rodents.
If that were the case, I would say don't worry about it until/unless you see the CHICKENS worrying about it
If they start to roost at the far end waaay far away from the popdoor, or start hunkering down on the bedding in a corner instead of going onto the roost, or if they seem unhappy, then you might want to curtain or close the popdoor. But really chickens are a lot cold hardier than most people give them credit for. If temperatures are above freezing, there is NO problem; if they are below freezing, then occasionally people have problems if there is chronic very very high humidity, but in dryish air most single-combed breeds are totally fine down to 20 or even 0 F (-10 or -20 C), just keep an eye on them.
(e.t.a - but even so, keep an eye out for signs that rodents are starting to chow down on the chicken feeder. Not only do you probalby not want to be the cause of a rat or mouse population explosion, it can also get really *expensive* feeding all those hungry mouths!)
Good luck, have fun,
Pat
